Sonocatalysis is a process that uses a photoactive material in the presence of ultrasonic irradiation but without the presence of light irradiation. Sonocatalytic degradation technology is highly efficient and operable due to the strong penetration of ultrasound waves in the pollutants during the degradation process (Xu et al., 2023a). Semiconductor materials such as TiO₂ have been employed for the sonocatalytic degradation of textile wastewater as shown in Figure 2 (Song et al., 2018). This process is largely based on the phenomenon of acoustic cavitation, which is defined as the growth and collapse of pre-existing microbubbles under the influence of an ultrasonic field in liquids. The formation of bubbles is due to the strong decline in local instantaneous pressure induced by strong ultrasound or by some hydrodynamic motion (Mondal et al., 2021; Liu et al., 2023). Under optimum conditions, the bubbles are likely to collapse very violently due to their inherent spherical geometry and the inertia of the surrounding liquid.

At the end of the violent bubble collapse, temperature and pressure inside the bubble significantly increase to more than 4000 K and 300 bar (1 bar = 105 Pa = 0.987 atm), respectively (Yasui, 2021; Xu et al., 2023b). This leads to the formation of reactive species as well as light emission known as sonoluminescence (SL), usually in the presence of a photoactive semiconductor material (Qiu et al., 2018). If the energy of light from sonoluminescence is greater than the band gap of the semiconductor, it causes it to be excited, resulting in the formation of electron-hole pairs (e^–/h⁺), thereby producing hydroxyl (HO^•) and superoxide (O_2^•− ) radicals at the valance and conduction bands, respectively (Thiemann et al., 2017). Additionally, the generated high temperature can possibly provoke the thermal excitation of the semiconductor material, leading to the generation of e^–/h⁺ pairs by thermal catalysis as well (de Andrade et al., 2021).

The generation of all of these ROS active in sonocatalysis is shown using the equations below: H 2 O + u l t r a s o u n d + c a t a l y s t →   H O • + H • (1) H O • + H •   →   H 2 O 2 (2) H • + O 2   →   H O 2 • (3) H O • + • O H   →   H 2 O 2 (4) H O 2 • + H O 2 •   →   H 2 O 2 + O 2 (5) H 2 O + • O H   →   H 2 O 2 + H • (6)

The formation of these highly reactive species has allowed the use of sonocatalytic processes in highly contaminated water. Thus, the process is efficient to treat wastewater or effluents containing an array of organic pollutants amongst others.

The photocatalysis process generally involves reduction and oxidation responses on the surface of photocatalyst material (Tahir et al., 2021). A schematic overview of photocatalytic degradation process is depicted in Figure 3.

Briefly, the absorption of light (photon energy higher than the band gap of the photocatalyst) by a photocatalyst creates holes (h⁺) on the valence band and electrons (e⁻) on the conduction band (Navidpour et al., 2023). The photo-created e^–/h⁺ pairs mediate the formation of species, like HO^• and O₂ ^•− from atmospheric oxygen and moisture (Fotiou et al., 2014). These species have been reported to have the potential to oxidize and break down organic pollutants, poisonous gas, and eliminate microorganisms from an aqueous environment (Zhao et al., 2020; Xu, 2021). Additionally, the photocatalytic process can also generate hydroperoxyl (HO₂ ^•) radicals which can aid the degradation of inorganic compounds present in industrial wastewater.

The generation of all of these ROS is shown on the equations below: S e m i c o n d u c t o r + h υ   →   S e m i c o n d u c t o r   h V B + + e C B − (7) h V B + + e C B −   →   h e a t (8) O H − + h V B +   →   H O • (9) H 2 O + h V B +   →   H O • + H + (10) H O • + H O • →   H 2 O 2 (11) e C B − + H + + O 2   → • H O 2 (12) H • O 2 + H • O 2   →   O 2 + H 2 O 2 (13) O 2 + e C B −   →   O 2 • − (14) O 2 • − + H + + H • O 2   →   O 2 + H 2 O 2 (15) H 2 O 2 + e C B −   →   H O • + O H − (16) H 2 O 2 + H O • →   H 2 O + H • O 2 (17) The high recombination rate of e^–/h⁺ pairs in the photocatalyst, as well as agglomeration result in loss of photocatalytic efficiency (Hayati et al., 2020). The combination of photocatalysis with sonocatalysis, however deals with these hindrances and results in highly efficient water treatment systems, hence, sonophotocatalysis is a more efficient water treatment technique (Wang and Cheng, 2023b).

Sonophotocatalysis generally entails the combination of light, ultrasound and a catalyst that work synergistically to accelerate the production of ROS, which are highly destructive against water pollutants (Malika and Sonawane, 2022). A schematic overview of sonophotocatalytic degradation process is depicted in Figure 4. Briefly, upon irradiation of a catalyst with ultrasonic waves, sonoluminescence results due to cavitation in the aqueous solution, causing pyrolysis of water molecules and generating highly reactive radicals (Wang and Cheng, 2023b). Additionally, light generated from sonoluminescence can excite the catalyst, leading to the formation of e^–/h⁺ pairs and subsequently ROS generation (de Andrade et al., 2021). Irradiation of the catalyst’s surface with light increases the generation of ROS as the h⁺ in the valence band react with water molecules adsorbed on the catalyst surface to generate HO^•, while the e^–generated in the conduction band react with dissolved oxygen to generate O₂ ^•−, HO^•, and H₂O₂ (Hanifehpour and Joo, 2018). These active species react with pollutants to generate different degradation intermediates and even mineralization products (H₂O and CO₂).

Therefore, while the use of light facilitates the degradation of pollutants through the generation of photo-induced ROS, ultrasonication enhance this process by stimulating the production of HO^• and assisting in reducing the recombination of photogenerated e^–/h⁺ pairs (Torres et al., 2008). Ultrasonication also helps with de-aggregation of the catalyst, which enhances its surface area and promotes mass transfer of pollutants between the liquid phase and the catalyst surface (Panda and Manickam, 2017). Moreover, combination of sono- and photocatalysis allows for efficient degradation of both hydrophobic and hydrophilic organic pollutants (Al-Musawi et al., 2021).

In addition to the irradiation sources (light and ultrasound), the overall efficiency of a sonophotocatalytic process also depends on the type of material (sonophotocatalysts) and their efficiency in generating sufficient ROS. Therefore, suitable sonophotocatalysts must exhibit the ability to respond to light and ultrasound irradiation while maintaining good chemical and photo-stability, good electronic properties and efficiency, as well as negligible toxicity and low cost. Semiconductor materials are commonly implemented due to their vital role in lowering the energy barrier for the formation of cavitation bubbles during the sonocatalysis process, while their light-harvesting capacity reveals the population of photogenerated e^–/h⁺ pairs during the photocatalysis process (Wang and Cheng, 2023b).

Selection of these semiconductors depends on their band gap energies (the minimum energy that is required to excite an e⁻ into the conduction band) which measures photoactivation potentials as shown in Table 1 (Dette et al., 2014; D’Amico et al., 2017; Djurišić et al., 2012; Doyan et al., 2021; González-Borrero et al., 2010; Przeździecka et al., 2021; Xu et al., 2018; Badar et al., 2012; Dalhat et al., 2022; Nurmalasari et al., 2020). Since most of them have large band gap energies (>2 eV), they are mainly activated using UV light. This does limit their real life applications as the UV regime is only a small fraction of the Sun’s energy (<10%) (Wang et al., 2016).

Various methods have been employed for the fabrication of these semiconductor sonophotocatalysts, some of the most common being; co-precipitation (El-Sawy et al., 2022), hydrothermal (Kumaresan et al., 2020), sol-gel (Leroy et al., 2020; Norabadi et al., 2020) sonochemical (Anandan and Ashokkumar, 2009) and the solvothermal (Rashid et al., 2023) method. The focus of this review is to showcase the advances that have been made in the application of sonophotocatalysis in water and wastewater treatment, thus these synthesis methods are not detailed.

Due to increased industrialisation and mass production of various products, countless toxic organic chemicals are readily discharged into water bodies, including pharmaceuticals and personal care products (PPCPs), dyes as well as pesticides (Rauf and Ashraf, 2009; Awfa et al., 2018; Fang et al., 2018). Polluted water with refractory organics is difficult to recycle and reuse (Zhang et al., 2020). In addition to sonophotocatalysis, several other treatment technologies [e.g., biodegradation, catalysis, coagulation and adsorption (Patsoura et al., 2007; Le Borgne et al., 2008; Wang et al., 2020; Xiao et al., 2021)] have been applied in the treatment of these hazardous pollutants. However, due to its high treatment efficiency, advances on the use of sonophotocatalysis as a treatment technique for these different organic pollutants is elaborated below.

The treatment of heavy metals in wastewater has proven to be a challenge that requires attention, as heavy metals are major wastewater pollutants that are not biodegradable, are toxic in the human body and can accumulate in the ecosystem (Vidu et al., 2020). Typical industrial methods employed in wastewater treatment, such as adsorption, chemical precipitation, ion exchange, ozonation and biological methods cannot efficiently reduce the metal concentration in water to within the regulatory standards effectively. This is because there is a considerable number of heavy metals in water that are complexed with organic chelating agents that come from textile, nuclear, and electroplating sources (Gao and Meng, 2021). Research on the sonophotocatalytic treatment of heavy metals is also very limited and is an area worth advancing, as the little work that’s been done has yielded promising results which demonstrate feasibility of this technique.

For instance, Chen et al. (2021) prepared zirconium–porphyrin metal–organic frameworks (MOFs) for the visible-light-driven sonophotocatalytic reduction of toxic Cr(VI) to Cr(III) in water. They found that the catalysts exhibited enhanced activities for Cr(VI) reduction compared with the photocatalytic process. Using fluorescence and UV–vis absorption spectra measurements, they were also able to deduce that the sonophotocatalytic process promotes the transfer of photoinduced electrons from the catalyst to Cr(VI), hence enhancement in the catalytic performance (Chen et al., 2021).

Chen et al. (2022) also prepared titanium–porphyrin MOFs and used them as visible-light-driven catalysts for the sonophotocatalytic reduction of Cr(VI). All the catalysts presented higher efficiency in the reduction of Cr(VI) to Cr(III) in aqueous solution when sonophotocatalytic treatment was employed than in photocatalysis. Sonophotocatalytic experiments and electron paramagnetic resonance measurement proved that the Ti-oxo chain units and porphyrin ligand in the structures of the catalysts existed as catalytic active centers for sonophotocatalytic reduction of Cr(VI). They also reported that photoluminescence and UV absorption spectra revealed that the synergy between photocatalysis and sonocatalysis strengthened the migration of photogenerated e⁻ from the catalyst to Cr(VI), which improved the activities of catalysts (Chen et al., 2022).

The disinfection of water is useful for the elimination of the pathogens that are responsible for waterborne diseases (Liu et al., 2021). Traditional disinfection methods typically entail the use of chlorine or other oxidative chemicals in water. There is however growing concern over the formation of harmful disinfection byproducts during chlorine- or chemical-based disinfection, which has heightened the need to develop alternative processes for water disinfection (Qiu et al., 2020).

Although still at its infancy stages, sonophotocatalysis has been explored as a viable method for the treatment of microbes, especially bacteria. The combined effects of ultrasonication and photocatalysis in microbes is depicted in Figure 7. The physical or mechanical consequences of ultrasound cavitation (such as shock waves and shear pressures) cause the mechanical rupturing of cell membranes and lead to cellular lysis (Dai et al., 2020). Additionally, the generated ROS from both sonocatalytic and photoinduced chemical reactions result in the damage of DNA and other intercellular components and they are then ejected from the cell due to cytoplasmic material leakage (Moradi et al., 2022).

Rahman et al. (2020) reported on the sonophotocatalytic treatment of Salmonella Typhimurium using Fe-doped ZnO nanoparticles (ZnO:Fe) wherein complete disinfection of the targeted bacteria was achieved within 45 min. The efficiency of ZnO:Fe nanoparticles was found to be higher than that of conventional sonophotocatalysts (ZnO and TiO₂). Their investigations also indicated that the HO^• and O₂ ^•− radicals could have been the key ROS interfering with the oxido-reductase protein system of the bacteria and hence hindered its metabolic activity. Additionally, they reported that the treatment process did not result in any process resistance, making it a good candidate for water disinfection (Rahman et al., 2023).

Mukherjee et al. (2022) reported on the efficiency of sonophotocatalysis in eradicating multi-drug resistant Klebsiella pneumonia. Hydrothermally synthesized CdS nanorods were applied as the sonophotocatalysts under blue light irradiation from a LED source in combination with low frequency ultrasonication for the complete disinfection of the coliform bacteria. Various reaction parameters including light intensity, sonication power, and catalyst dose were found to affect the disinfection capability. Following optimisation, the disinfection strategy was able to handle 106 CFU/mL of K. pneumonia efficiently in 20 min. The sonophotocatalyst demonstrated good reusability and stability. They elucidated that sonophotocatalysis is a viable disinfection technology which utilises the release of ROS and mechanical shear to induce bacterial disinfection (Mukherjee et al., 2022).

Drosou et al. (2010) reported on the efficacy of heterogeneous photocatalysis and sonophotocatalysis induced by UV-A irradiation and low frequency (24–80 kHz) ultrasound irradiation in the presence of TiO₂ and peracetic acid (PAA) as an additional disinfectant to inactivate Escherichia coli in water. PAA-assisted UV-A/TiO₂ photocatalysis generally led to nearly complete bacterial inactivation in 10–20 min of contact time with the extent of inactivation depending on the photocatalyst type and loading (in the range 100–500 mg L⁻¹) and PAA concentration (in the range 0.5–2 mg L⁻¹). PAA-assisted UV-A/TiO₂ sonophotocatalysis however showed complete bacterial inactivation with shorter treatment times and lower PAA doses and was therefore reported as the most efficient treatment technique (Drosou et al., 2010).

One of the prevalent strategies employed for further enhancing the efficiency of sonophocatalytic treatment of water pollutants is the modification of the sonophotocatalysts. Modification of properties such as physical, electronic and magnetic properties in nanometer scale help with enhancement the sonophotocatalytic activity. This can be achieved by various methods including; doping the sonophotocatalysts with metals or non-metals (Tabasideh et al., 2017; Rajoriya et al., 2019). The dopants help increasing the absorption band width into the visible region as well as reducing the hole–electron recombination (Chakma and Moholkar, 2015). This corroborates with Paul et al. (2017) who evaluated the sonophotocatalytic efficiency of pure and Ag doped flower-like hierarchical molybdenum oxide nanorods for the degradation of Methylene Blue dye under diffused sunlight. They found that the optical band gap energy decreased with Ag doping while the degradation efficiency was increased (Paul et al., 2017).

Anomalies do occur though, for instance, Ikram et al. (2021) prepared various concentrations of Mg-doped ZnO nanorods which they applied in the photocatalytic, sonocatalytic and sonophotocatalytic degradation of a mixture of Methylene Blue and ciprofloxacin. As shown in Figure 8A, they observed a shift in absorption upon doping which they attributed to oxygen deficiency, particle size effect and grain structure defects. They extracted values from Figure 8A to calculate optical band gap (E.g.,) of ZnO (using Tauc plots), which increased from 3.32 to 3.72 eV with increased Mg doping (Figure 8B). They explained the blueshift in E.g., based on the Burstein–Moss effect phenomenon. They explained that in the metal oxide method, particle size reduction can result in a blueshift of the band gap due to the quantum confinement effect (QCE). They further elaborated that doping may also affect local symmetry and generate lattice defect centers that change the structure of the band and induce significant shifts in optical properties. Nonetheless, the experimental results however showed improved degradation performance by Mg-doped ZnO nanorods relative to the bare ones (Ikram et al., 2021).

Another promising strategy for enhancing sonophotocatalysis is by coupling the nanoscale semiconductor sonophotocatalyst with another lower band gap semiconductor i.e., creating heterojunctions (Zhang et al., 2021). The close contact of semiconductors with different Fermi levels leads to the formation of an internal electric field. This electric field can direct the movement of electrons and holes through the interfacial zone (Moradi et al., 2022). For instance, Guo et al. (2021) investigated the influence of the heterojunction by comparing the sonophotocatalytic capabilities of ZnO and ZnO/ZnS core–shell nanorod arrays in the degradation of Methylene Blue. As depicted in Figure 9, they observed that the nanorods with the heterojunction i.e., ZnO/ZnS core–shell had a higher degradation efficiency (45.4%) than ZnO (25.1%). They also studied the morphological influence in the heterojunctions by comparing ZnO/ZnS nanorods and ZnO/ZnS nanotubes which further showed that the latter had a higher capability of 63.3%. They ascribed the improvement to the coupling effect of the enhanced piezoelectric field and the reduced migration distance, which suppresses the recombination of photoexcited e^–/h⁺ pairs while transforming the morphology from nanorod to nanotube (Guo et al., 2021).

The sonophotocatalysts can also be modified by dye photosensitisation i.e., conjugation of the semiconductor sonophotocatalyst to a dye (Kaur and Singh, 2007; Mkhondwane et al., 2023a). This results in composite materials with two light absorbers, wherein the dye absorbs visible light, which is beneficial as most of the solar irradiation is in the visible region. The synergistic interaction of a dye with a semiconductor photocatalyst is depicted in Figure 10. The excited dye causes an influx of electrons in the conduction band of the semiconductor, thereby enhancing the generation of ROS which can then decompose a range of pollutants. Moreover, ultrasonic cavitation may cause the pyrolysis of water molecules in contact with the surface of the sonophotocatalyst, thereby further enhancing the generation of ROS (de Andrade et al., 2021).

Mkhondwane et al. (2023) investigated the use of two Zn phthalocyanines as photosensitisers for TiO₂ fibers in sonocatalytic, photocatalytic, and sonophotocatalytic degradation of Rhodamine 6G. They found that the ZnPc functionalized fibers displayed higher activity than their corresponding bare ZnPc analogous. Additionally, the sonophotocatalytic process displayed higher degradation aptitude than sonocatalytic and photocatalytic processes (Mkhondwane et al., 2023b). Moreover, Hakki and Allahyari (2022) prepared Bi₂O₃-ZnO photocatalysts with various amounts of copper phthalocyanine as a photosensitiser for the sonophotocatalytic degradation of Methyl Orange dye. They compared the sonophotocatalytic degradation of Methyl Orange under simulated solar light with sole photocatalysis and sonolysis, and the results showed that the sonophotacatalytic degradation was the most efficient (Hakki and Allahyari, 2022).

The alteration and optimisation of the actual sonophotocatalytic process can also drastically improve the wastewater treatment results. For instance, using gases (N₂, Ar, O₂) to purge the sonophotocatalytic reaction enhances the performance of the system by generating high-potential bubble formation sites. This then results in an increased rate of cavitation and greatly influences the amount of produced H₂O₂, which can actively participate in and contribute to the pollutant degradation process (Moradi et al., 2022).

The addition of oxidation agents (O₃ and H₂O₂) has also been reported to enhance sonophotocatalytic efficiency (Joseph et al., 2009; Sathishkumar et al., 2016). The addition of H₂O₂ can cause the release of a large number of free and adsorbed radicals in the medium and it can act as an electron trap to suppress e^–/h⁺ recombination (Moradi et al., 2022). For instance, Wei et al. (2021) observed an enhancement in the sonophotocatalytic performance of H₂O₂-assisted titania based system for degradation of Acid Orange 7 (Wei et al., 2021). Ozonation on the other hand can be utilised to enhance the production of reactive species either alone or in combination with ultrasound and or light irradiation (Kohantorabi et al., 2022).

Lastly, Fe-containing catalysts integrated with light and ultrasound improve the pollutant removal efficiency due to enhanced synergistic interaction in a process called sono-photo-Fenton catalysis (Moradi et al., 2020; Qi et al., 2020). The reaction of ferrous ions with in situ produced H₂O₂ during sonophotocatalytic reactions provides free additional radicals that facilitate the degradation of dissolved contaminants (Moradi et al., 2022). Unfortunately, the catalyst can suffer from poisoning during the oxidation process, due to the adsorption of reaction intermediates on its surface, thereby leading to a progressively inactive surface (Zhong et al., 2011). Segura et al. (2009) investigated the sono- and photo-Fenton processes separately as well as the combined systems (sono-photo-Fenton) in order to evaluate of possible beneficial effects on the use of coupled systems in the degradation of phenol. They observed enhanced activity for the sequentially-integrated sono–photo-Fenton process which resulted in a total phenol degradation and ca. 90% TOC reduction. They attributed the effective degradation of the sequential sono–photo-Fenton process to the benefits of ultrasound for the partial degradation of amenable aromatic compounds towards further oxidized by-products along with the fragmentation of catalyst to finer particles that produces more activity in photo-Fenton systems (Segura et al., 2009).